Electric power cables

ABSTRACT

An electric power cable of the kind having at least one metallic conductor, optionally a conductor screen of semiconductive material, and insulation all enclosed in an earth screen formed by wires or narrow tapes and a sheath formed by polymeric material is characterised by a sheath construction including a first extruded polymeric layer which encloses the earth screen of the cable without filling the gaps between the components thereof (the wires or strips), a second extruded polymeric layer enclosing the first and between them a moisture barrier laminate having a metal foil layer coated on one of its major faces with a polymeric material compatible with the material of the extruded layer it contacts and bonded to that extruded layer only. That increases the resistance of the cable to impact, compared with constructions in which a moisture barrier foil layer is applied directly over the screen wires, and increases flexibility and reduces cost compared with constructions in which the foil is bonded to both extruded layers.

BACKGROUND OF THE INVENTION

This invention relates to electric power cables of the kind having atleast one metallic conductor, usually a conductor screen ofsemiconductive material, and insulation all enclosed in an earth screencomprising wires or narrow tapes and an external sheath comprisingpolymeric material which will usually (but not necessarily) form theexposed surface of the cable.

In most environments, it is desirable, if not essential, that the sheathis more effectively watertight than can be achieved with polymericmaterial alone, and attempts have thus been made to follow the practiceused in communications cables of including a metal/plastics laminatefoil beneath the main body of the sheath, so as to become bonded to it,normally when the main polymeric body is extruded. It is difficult, withsuch structures, to design a sheath in which the laminate foil remainsintact when the cable is subjected to the impact tests specified byStandards Authorities and customers, as the laminate tends to be driveninto the gaps between the wires or tapes of the screen and to splitalong resulting crease lines.

In our prior application GB9620394.8 which is related to WO98/14960 andU.S. Ser. No. 09/230,565 cable characterised by a sheath constructioncomprising a first extruded polymeric layer which encloses the earthscreen of the cable without filling the gaps between the componentsthereof (the wires or strips), a second extruded polymeric layerenclosing the first and between them and bonded to both of them amoisture barrier laminate comprising a metal foil layer coated on eachof its major faces with a polymeric material compatible with thematerial of the extruded layer it contacts.

Surprisingly, we have now discovered that it is possible to achieveadequate performance in impact tests in a similar construction with themoisture barrier foil bonded to only one of the two extruded layers.This offers an improvement in flexibility as well as an appreciable costsaving (as the foil needs to be coated on only one face), though thereis some diminution of longitudinal water-blocking effect.

SUMMARY OF INVENTION

The cable in accordance with the present invention is thus characterisedby a sheath construction comprising a first extruded polymeric layerwhich encloses the earth screen of the cable without filling the gapsbetween the components thereof (the wires or strips), a second extrudedpolymeric layer enclosing the first and between them a moisture barrierlaminate comprising a metal foil layer coated on one of its major faceswith a polymeric material compatible with the material of the extrudedlayer it contacts and bonded to that extruded layer only.

Preferably the metal foil layer is coated on only one face, and this isnormally sufficient of itself to prevent adhesion to the other extrudedlayer of the sheath, but as an additional precaution (or instead) asuitable parting agent may be applied to one or both of the surfacesthat it is desired should not bond. Graphite applied from aqueoussuspension is preferred as parting agent, but talc (“chalk”) or asuitable oil might be used in some cases.

Preferably the foil is bonded to the outer extruded layer of the sheathand prevented from bonding to the inner layer.

Preferably the extruded polymeric layer to which the foil is to bebonded is of the same polymeric material as the coating on the foil; theother may but need not be of a substantially different composition.Options include the usual cable sheathing materials, namelypolyethylene, polyvinyl chloride, “LSF” compositions based onethylenevinyl acetate copolymer and ethylene-propylene rubbers (in eachcase with conventional additives).

The laminate is preferably longitudinally applied, but could behelically lapped if desired.

Preferably the insulation of the cable is of a polymeric material, suchas crosslinked polyethylene or ethylenepropylene rubber, and in thiscase the conductor screen is preferably of carbon-loaded polymericmaterial and the earth screen preferably includes an inner layer ofcarbon-loaded conductive polymeric material, in addition to the wires orstrips of metal.

Preferably the gaps between the wires or strips of the earth screen arewaterblocked by applying water-swellable tapes under and/or over them,or in other known ways. Preferably at least one semiconductivewater-swellable tape is applied under the wires or tapes and at leastone insulating water-swellable tape over the top of them. Awaterblocking tape applied over the wires or tapes may facilitateensuring that the first extruded layer of the sheath does not fill thegaps between them. Other suitable tapes could be used for this purposeif desired.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further described, by way of example, withreference to the accompanying drawings in which

FIG. 1 is a diagrammatic cross-section of one form of single-core cablein accordance with the invention and

FIG. 2 is an enlargement of the portion of FIG. 1 enclosed by the boxII.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

The cable of the example has a stranded plain annealed copper conductor1, a semiconductive conductor screen 2 applied over a binder tape 2A,crosslinked polyethylene insulation 3 and a dielectric screen comprisingan inner semiconducting layer 4 (which may be “fully bonded” or“strippable”) and a layer of copper wires 5, all of which areconventional. A semiconducting water-swellable tape 6 under the wiresand an insulating water-swellable tape 7 over them provide longitudinalwater-blocking for the gaps between the wires. In accordance with theinvention, this conventional part of the cable 1 is enclosed in a sheathcomprising a first extruded layer of polyethylene 8, a longitudinallyapplied tape 9 with a base layer 10 of aluminum coated on only one side,applied outwards, with polyethylene 11, and a second extruded layer ofpolyethylene 12. The conditions and die dimensions for the extrusion ofthe first layer of the sheath are such that the layer is “tubed” anddoes not embed the wires of the screen to any substantial extent. Thetemperature at which this second layer is applied is high enough toensure that the laminate is adhesively bonded to the second polyethylenelayer but, in the absence of a coating on the inside, it does not bondto any significant extent to the first layer.

In a particular example constructed as just described, a copperconductor of 300 mm² (made up of 59 round wires each 2.6 mm in diameter)is covered with a conventional semiconducting screen material to anominal radius of 25.3 mm. The crosslinked polyethylene insulation has aradial thickness of 18.9 mm and a tensile strength of 17 MPa measuredaccording to ISO 527 in the fully-crosslinked state using an extensionrate of 250 mm/min and the screen comprises a layer 1.0 mm thick of aconventional fully-bonded screen composition and 69 soft annealed copperwires each 1.78 mm in diameter. The inner and outer layer of the sheathhave thickness of 2.9 mm and 7.4 mm respectively and both are made of athermoplastic polyethylene cable-sheathing composition with a tensilestrength of 20 MPa measured according to ASTM D638, while the laminatehas an aluminium base layer 0.2 mm thick and a coating of polyethylene0.050 mm thick on one side; it is 230 mm wide and applied longitudinallywith the coating outwards and so that its edges overlap by a minimum of5 mm. This cable reliably passes the impact test specified by CIGREspecification WG 21.14 (Elektra no.141, April 1992) using an impactingweight of 5 kg in the form of a 90° wedge with 2 mm tip radius droppedfrom a height of a metre.

What is claimed is:
 1. An electric power cable of the type having atleast one metallic conductor, a conductor screen of semiconductivematerial, and insulation, and an earth screen enclosing the conductor,conductor screen and insulation, the earth screen having wires with gapstherebetween, the electric power cable comprising an external sheathincluding a first extruded polymeric layer which encloses the earthscreen of the cable without filling the gaps, a second extrudedpolymeric layer enclosing the first polymeric layer and a moisturebarrier laminate located between the first and second polymeric layers,said moisture barrier laminate including a metal foil layer and apolymeric material coating on at least one surface of the metal foillayer, the polymeric material contacting one of the first and secondextruded polymeric layers and compatible with a material of thecontacted one of the first and second extruded polymeric layers andbonded to the contacted one of the first and second extruded polymericlayers only.
 2. The cable as claimed in claim 1 in which the polymericmaterial is coated on only one surface of the metal foil layer.
 3. Thecable as claimed in claim 2 in which a parting agent is applied to oneor both of the surfaces of the metal foil layer that are not bonded. 4.The cable as claimed in claim 1 in which a parting agent is applied toone surface of the moisture barrier laminate that is not bonded.
 5. Thecable as claimed in claim 1 in which the metal foil layer is bonded tothe second extruded layer of the external sheath and not to the firstextruded layer.
 6. The cable as claimed in claim 1 in which the moisturebarrier laminate is longitudinally applied.
 7. The cable as claimed inclaim 1 in which the insulation of the cable is of a polymeric material.8. The cable as claimed in claim 1 in which the gaps between the wiresof the earth screen are waterblocked.
 9. The cable as claimed in claim 1in which the gaps between the wires of the earth screen are waterblockedby applying water-swellable tapes under or over the wires.
 10. The cableas claimed in claim 9 in which said water-swellable tapes include atleast one semiconductive water-swellable tape applied under the wiresand at least one insulating water-swellable tape applied over the top ofthe wires.
 11. The cable of claim 1, wherein the polymeric materialcontacts the second extruded layer and a second surface of the metalfoil layer contacts the first extruded layer.
 12. The cable of claim 1,the metal foil layer having a second surface contacting the other one ofthe first and second extruded layer, but not being bonded to that layer.13. An electric power cable of the type having at least one metallicconductor, a conductor screen of semiconductive material, an insulation,and an earth screen enclosing the conductor, conductor screen andinsulation, the earth screen having tape strips with gaps therebetween,the electric power cable comprising an external sheath including firstextruded polymeric layer which encloses the earth screen of the cablewithout filling the gaps, a second extruded polymeric layer enclosingthe first polymeric layer and a moisture barrier laminate locatedbetween the first and second polymeric layers, said moisture barrierlaminate including a metal foil layer and a polymeric material coated onat least one surface of the metal foil layer, the polymeric materialcontacting one of the first and second extruded layers and compatiblewith the material of the contacted extruded layer and bonded only to thecontacted extruded layer.
 14. The cable as claimed in claim 13 in whichthe polymeric material is coated on only one surface of the metal foillayer.
 15. The cable as claimed in claim 13 in which a parting agent isapplied to one surface of the moisture barrier laminate that is notbonded.
 16. The cable as claimed in claim 13 in which the metal foillayer is bonded to the second extruded layer of the external sheath andnot to the first extruded layer.
 17. The cable as claimed in claim 13 inwhich the moisture barrier laminate is longitudinally applied.
 18. Thecable as claimed in claim 13 in which the insulation of the cable ispolymeric.
 19. The cable as claimed in claim 13 in which the gapsbetween the tape strips of the earth screen are waterblocked.
 20. Thecable as claimed in claim 13 in which the gaps between the tape stripsof the earth screen are waterblocked by applying water-swellable tapesunder or over the tape strips.
 21. The cable as claimed in claim 20wherein the water-swellable tapes include at least one semiconductivewater-swellable tape applied under the tape strips and at least oneinsulating water-swellable tape applied over the top of the tape strips.22. The cable of claim 13, wherein the polymeric material contacts thesecond extruded layer and a second surface of the metal foil layercontacts the first extruded layer.
 23. The cable of claim 13, the metalfoil layer having a second surface contacting the other one of the firstand second extruded layer, but not being bonded to that layer.